Sample preparation is an important but often neglected step in chemical analysis. The importance of an efficient sample preparation technique is even greater when dealing with trace and ultra-trace levels of target analyte(s) dispersed in complex sample matrices e.g., environmental, pharmaceutical, food, and biological samples. These samples are not generally suitable for direct injection into the analytical instrument for chemical analysis. This incompatibility is attributed to three main factors. First, the matrix ingredients may inflict a detrimental effect on the performance of the analytical instrument, or they may interfere with the analysis of target analytes. Second, the concentration of the target analyte(s) in the sample matrix may be too low to be detected by the analytical instrument. Third, the sample matrix may be incompatible with the analytical instrument. For example, aqueous solution containing the target analyte(s) cannot be injected into a gas chromatograph (GC) unless a solvent exchange (e.g., from water to an organic solvent) is carried out. As such, the primary objective of sample preparation is to isolate and concentrate the target analyte(s) from various sample matrices (biological, environmental, pharmaceutical, food etc.) into a new solvent/solvent system and to minimize matrix interference so that the cleaner analyte(s) solution can be introduced into the analytical instrument for separation, identification, and quantification without having a detrimental impact on the performance of the analytical instrument.
Classical sample preparation techniques such as liquid-liquid extraction (LLE) and solid phase extraction (SPE) are still among popular choices for environmental, pharmaceutical, toxicological, forensic and biological sample preparation. However, these procedures are time consuming, laborious, multi-step and utilize large volumes of toxic and hazardous organic solvents, leading to additional post-extraction steps such as solvent evaporation followed by sample reconstitution in a suitable solvent.
The solid-phase microextraction (SPME) is considered to be a new mode of sample preparation distinct with solvent-free/solvent-minimized microextraction, miniaturization, and automation. Due to the substantial advantages over conventional solvent-intensive sample preparation techniques, SPME has gained enormous popularity. However, SPME has a number of shortcomings. One major shortcoming of SPME (fiber format) is the miniscule amount (typically ˜0.5 μL) of sorbent loading, which often results in poor extraction sensitivity. The low extraction sensitivity of fiber-SPME prompted the invention of a number of microextraction techniques with higher sorbent loading including in-tube SPME, Stir Bar Sorptive Extraction (SBSE), Micro Extraction by Packed Syringe (MEPS), rotating-disk sorptive extraction (RDSE), and thin film microextraction (TFME).
SPME and its different formats, modifications and implementations are generally governed by thermodynamic and kinetic criteria. Thermodynamic properties determine the maximum amount of analytes that can be extracted by a given mass of sorbent under a specific set of extraction conditions. Since higher volume of sorbent loading allows a higher amount of target analytes to be accumulated by the sorbent when adequate time is allowed to reach the extraction equilibrium, sorbent loading is directly related to extraction efficiency. Kinetics of the rate of extraction dictates the time required to reach the extraction equilibrium. The faster that extraction equilibrium is achieved, the higher the throughput in the analytical lab. As a result, there is a pressing demand to develop new microextraction techniques that can simultaneously satisfy the required sensitivity while permitting the shortest possible sample preparation time.
Critical evaluation of different microextraction systems reveals that shortcomings of all microextraction systems originate from: (1) coating technology used for immobilizing the sorbent on the substrate surface; and (2) the physical format of the extraction system that defines the primary contact surface area (PCSA) of the device where the extraction medium makes direct contact with the sample matrix containing the analytes. Therefore, if a sample preparation technique is to be highly sensitive as well as fast, both the coating technology and the primary contact surface area have to be augmented.
A majority of the shortcomings suffered due to the sorbent coatings used in commercially available microextraction technologies, such as, bleeding, washing away with organic solvent, long extraction equilibrium time, limited selectivity, extraction reproducibility, and swelling of the sorbent originate, primarily result from the process of immobilizing the organic polymer on the substrate surface. These coatings are generally created by physical deposition, followed by free-radical cross-linking reactions. The lack of chemical bonding between the polymer sorbent and the substrate is believed to be the primary cause of these coating-related problems. A number of alternative coating techniques have been proposed including: physical deposition; electrochemical deposition of conducting polymers; gluing with adhesives; and sol-gel column technology. Sol-gel column technology has proven to be the most convenient and versatile. In addition to the convenience of the coating process, sol-gel technology opens up the possibility of making multi-component materials that can be conveniently used to customize the surface morphology, selectivity, and affinity of the sorbent. The sorbent coating created by sol-gel technology is highly porous and is chemically bonded to the substrate. These coatings demonstrate remarkable thermal, solvent, and chemical stability. Due to its inherent porosity, a thin film of sol-gel coating can extend equivalent or higher sensitivity than commercially available, thick SPME coatings. The high porosity of the sol-gel coating also makes it possible to reach extraction equilibrium in a fraction of the time that is required by commercial SPME fibers.
Although tremendous efforts have been made to increase the sensitivity of the microextraction techniques by merely increasing the sorbent loading via expanding the surface area of the substrates, major challenge remains extracting target analytes directly from a complex sample matrix containing high volume of matrix interferents, such as, soil particulates, debris, biomasses, cells, proteins, fats, and tissues. These interferents may irreversibly adhere to the extraction sorbents of the device, leading to permanent damage and efficiency loss of the extraction device. Such samples require pretreatment, such as, filtration/centrifugation/protein precipitation or a combination thereof, to obtain a clean, particulate free, aqueous sample prior to extracting the analyte of extraction using SPE, LLE, SPME, SBSE, TFME and different modified techniques. Pretreatments including filtration and/or centrifugation involve the use of filter papers with sample transfer between one or more glass or plastic containers that results in moderate to significant analyte loss; depending on the physicochemical characteristic of the analyte and the type of filter paper/container used. Biological samples, e.g., blood, plasma, and urine, inevitably require protein precipitation to obtain a clean, particulate free environment prior to the extraction of the target analyte. Protein precipitation can result in significant analyte loss, especially if the analytes are medium polar or nonpolar analytes. Analyte losses during sample preparation pose serious legal, medical, and environmental ramifications when dealing with trace or ultra-trace levels of concentrations. Therefore, sample preparation should offer high analyte retention capacity, possess fast extraction kinetics, and should be capable of extracting a sample even when the environment sampled contains high volume of matrix interferents.
Sol-gel coatings chemically bind to many substrates, such as silica, when the gel is formed from the sol solution in the presence of the substrate. Because of the wide variety of possible sol components, a large number of sorbents for SPME are possible. Sol-gel monolithic beds are capable of achieving very high sample pre-concentration factors. Sol-gel technology has resulted in surface-bonded sorbent coatings on unbreakable fiber materials, e.g., Ni—Ti, stainless steel, titanium, and copper, and on substrates of different geometrical formats such as planar SPME (PSPME), and membrane SPME (MSPME). A wide variety of sol-gel silica, titania, zirconia, alumina, and germania-based precursors are commercially available. Additionally, a wide range of sol-gel reactive organic ligands are available to design hybrid organic-inorganic sol-gel coatings that can be used to target a particular analyte or sample matrix with improved selectivity, sensitivity, extraction phase stability and performance.
However, most microextraction devices are not designed for direct contact with the sample matrix when the matrix contains a high volume of particulates, debris or other matrix interferences that may cause irreversible damage to the sorbent coating. Hence, there remains a need for a simple device that can be placed in a sample matrix, easily retrieved, and capable of placing into an analytical device's sample introduction equipment for rapid sampling and reliable analysis of target analytes resident in a wide scope of environments.